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The dinosaurs reign on earth for hundreds of millions of years was more by luck than any innate superiority, says a UK study.

The study, the first of its kind to compare dinosaurs with the ancient ancestors of modern crocodiles, appears in today's issue of Science.

Led by Stephen Brusatte of the University of Bristol, the researchers found that there was no evolutionary feature that made the dinosaurs any better than other animals living at the time.

Instead, they believe dinosaurs better survived the late Triassac (230 to 190 million years ago) extinction period than the crurotarsans - a mostly extinct group that included the ancestor of the modern crocodile.

"If any of us were standing by during the Triassic and asked which group would rule the world for the next 130 million years, we would have identified the crurotarsans, not dinosaurs," says Brusatte.

Brusatte and colleagues analysed a large collection of bones from 64 species of dinosaurs and crurotarsans.

They looked at 437 skeletal features to determine the rates of evolution between dinosaurs and crurotarsans and found that both groups evolved at the same rate.

The researchers say that if dinosaurs were more superior, they should have evolved faster. They also found that crurotarsans had about twice the disparity during that period.

The research builds on previous studies showing crurotarsans were more numerous and in some cases more diverse than the dinosaurs in the late Triassic.

Random luck

"Why did crurotarsans go extinct and not dinosaurs? We don't know the answer to that, but we suspect that it was nothing more than luck, plain and simple," says study co-author Professor Michael Benton of the University of Bristol.

Benton says most people see evolution as progressive, akin to improvements in car technology, which makes it hard to accept that dinosaurs achieved their dominant position on earth largely by chance, just as mammals did when the dinosaurs became extinct.

Australian vertebrate palaeontologist Dr Steve Salisbury, from the University of Queensland in Brisbane, who was not part of the study, says the research highlights that a lot of evolution is due to chance, perhaps even more than natural selection does.

He says it isn't clear why the dinosaurs were the lucky ones.

"A lot of non-dinosaur archosaurs were among those hit really hard by whatever the [extinction] event was," says Salisbury.

"Dinosaurs are just one of several groups that luckily made it through whatever knocked out most of the rest of the animals at the end of the Triassic."

That mass extinction event may have been a fortunate turn of events for humans.

If it weren't for the Triassic extinction "we might have had huge, gigantic crocs and flying crocs - that's where the crurotarsons were headed," Salisbury says.